It is necessary to blend air from the compressor of an aircraft-mounted gas turbine engine in order to drive pneumatically powered aircraft accessories and to provide pressurised cabin air. Usually the air is bled from the higher pressure compressor regions and as a consequence is often at relatively high temperature. The high temperature of the air can be a problem when it is directed through pipes contained within the main structure of the aircraft, in particular its wings. Moreover, the majority of the air bled from the engine is used for purposes which do not require the air to be hot. Conventionally therefore, the air is cooled immediately after it has been bled from the engine compressor.
In a ducted fan gas turbine engine, the bled air is directed through pipes which have to pass across the fan duct en route to the main aircraft structure. Typically, therefore, the pipes are located within the fairing which encloses the engine mountings, fuel pipes etc and extends across the upper region of the fan duct. The cooling of the bled air takes place within this fairing in a heat exchanger commonly referred to as a precooler. Air tapped from the fan exhaust air flowing through the fan duct is directed to the precooler in order to provide cooling of the bled air. The tapped fan air usually enters the fairing through an inlet provided in the outer surface of the fairing. However control must be exerted over the tapped air flow. This is because the temperature of the air bled from the compressor varies in accordance with the operation of the engine: The lower the speed of aircraft the hotter the supply of compressed air because of the operation of the bleed system which is required to supply high pressure air against falling engine thrust/speed. In the past this control has been provided by an on/off valve located within the fairing. When the air bled from the compressor is relatively cool, the valve is switched off so that no fan air is tapped off and directed to the precooler. However when the engine is operating under conditions which result in the bled air being too hot, the valve is opened to permit a flow of fan air to the precooler.
As the power outputs of ducted gas turbine engines have risen, the temperatures of the air within the compressors of those engines have risen correspondingly. This has been even more pronounced with the trend towards fitting large aircraft with smaller numbers of engines. Thus whereas in the past, larger aircraft have been fitted with four engines, there is now a trend towards fitting new aircraft in this size category with three or two high power engines. As a result of this, the demand for the cooling of the air bled from the engine has increased. It is now frequently desirable that the bled air should be cooled over a larger proportion of engine operating conditions and that, in the interests of efficiency, the flow of tapped fan air used for cooling should be capable of being varied over a range of flow rates.